‘Rain Gardens’ is a term used for a type of green infrastructure that is made up from plants that are fed by the rain runoff from roofs etc. They are usually small depressions in the ground on mainly residential sites but can vary in size and are sometimes also called ‘bioretention cells’. They aid infiltration by slowing the water down and increasing soil permeability, as well as taking up the water themselves. In doing so, they also remove sediment and pollutants to a certain degree. However, they can also be designed to remove pollutants efficiently by incorporating a waterlogged area.
Increase attractiveness of a place and can so improve use, however little potential to increase exercise rates etc due to inaccessibility (2,7)
Vegetation and soil can trap air pollutants and dust. (16)
Infiltration of up to 90% of annual roof runoff but large events cannot be fully adsorbed – a raingarden with 20% of the roof area should infiltrate this amount of runoff from the roof. Even to larger storms (e.g. 50yr RP) raingardens can provide peak flow attenuation of 20%. However, small catchment areas mean that singular raingardens don’t contribute greatly to overall reduction. (1,2,6,10,12,13, 16)
Rain Gardens are unlikely to contribute to reducing fluvial flooding.
Carbon storage and sequestration possible, potential to improve UHI effect but very small scale. (9,17)
Native plants and shrubs can provide habitats for insects, if right plants are selected can be very good for pollinators, due to small scale benefit is unlikely to be high. (2,3,7,9)
Potential to recharge groundwater.
Removal of sediments and nutrients (dependent on design) off up to 99%. Metal reduction varies between 30 and 99%. Only nitrogen reduction is usually less – improves with the introduction of a permanently saturated zone! After dry periods heavy rainfall can result in high pollutant load to receiving watercourse. (1,2,4,6,7,9,10,11,12,16)
Very high potential to be designed to be aesthetic landscape features. (1,2,5,7,8, 12)
Potential for gardening activities, in bigger systems planting of fruit bearing shrubs might give opportunity for farming related activities. (2, 12)
Rain gardens could potentially contribute to increased property values. (1)
Taking up water from their own area and surrounding areas can help reduce the risk of flooding and the extent of flooding on a larger scale.
Considering the Bigger Picture
Rain Gardens act as source control and infiltration devices. This means they receive rain immediately after it falls and are best used upstream of any pond, wetland or similar structure, being especially effective for smaller, but more frequent events, with a smaller pollutant load.
Opportunities for planting Rain Gardens include planters in built up areas, combining them with Rainwater Harvesting (e.g. as a receptor for overflow from water butts) or trees by planting them in tree pits.
On the left, you can find an example of how different interventions can be incorporated into the urban landscape.
To provide a comprehensive treatment and management of surface water, rain gardens should be seen within the wider landscape. While they are able to intercept rainfall before it becomes runoff, it is important to understand that their ability to take up existing runoff and infiltrate it is limited, however their cumulative impacts on an area should not be underestimated.
Rain Gardens can be used in many shapes and sizes and there are many opportunities to incorporate them into the urban landscape. They could be part of roundabouts, parking bays or along roadsides, or they could be incorporated into existing green spaces to provide further treatment of runoff and attentuation.
£20-270+/m2 dependent on size and context (£££ – low) very variable due to high variability of design and context. (2,12, 15)
Residential, Industrial, Retrofit. High hydraulic conductivity must be given and a drain for exceedance flows must be established (e.g. drain into existing sewer system). In built up areas, rain gardens can also be established in planters. Ideally between 3-5m wide, length adjusted to suit slope. (2,13,14,15)
low dependent on context but mainly litter/sediment removal. Plants need to be selected to endure waterlogged as well as dry conditions. (2, 8, 14,15)
Trade-offs and Potential Dis-services
if not selected adequately, plants used might need replacing and decrease aesthetic value of the planting. Could also attract pests.
Overflow from clogging can occur and reduce effectiveness of the intervention and potentially distribute first flush pollutants.
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